1. Field of the Invention
The present invention relates generally to the art of drilling large diameter shafts in the earth and, more particularly, to the cleaning of the shaft bottom during the drilling operations.
2. Description of the Prior Art
Large diameter shafts are basically utilized in the mines for rescue, ventilation, ore and coal hoisting, and general access purposes. In recent years, large diameter shafts have been utilized for emplacing nuclear devices.
Large diameter shafts are usually drilled with rotary flat bottom bits which comprise a main bit body having a plurality of rotatively supported roller cutters attached thereto. The cutters function to contact and disintegrate the earth formation at the bottom of the shaft, thereby creating cuttings or chips at that location.
Various circulation systems utilizing water or drilling mud have been used to flush the earth cuttings away from the shaft bottom. Such circulation systems are usually classified as either direct or reverse circulating systems. In the direct circulating systems, drilling fluid is pumped down the center drill column, across the face of the drill bit at the bottom of the shaft, and up through the annulus of the shaft. In the reverse circulating systems, the drilling fluid is pumped down the shaft annulus to the bottom of the shaft, across the face of the drill bit and up through the center drill column back to the surface. In both systems, when the drilling fluid transporting the chips and cuttings reaches the surface, the fluid is usually pumped through various separating and cleaning devices to separate the cuttings, silt, gas and other materials from the drilling fluid in order to enable the cleaned drilling fluid to be recycled for further use.
Although present big hole rotary drilling systems work adequately, present day systems have several shortcomings which impair their performance. One major shortcoming is that present day circulating systems are inefficient for transporting the drilled cuttings across the bottom of the shaft and away from the cutting action of the drill bit. As a result, the rock chip cuttings are reground before being removed from under the bit. It has been determined that the regrinding of these chips to a fine size requires large amounts of energy, decreases the penetration rate, and decreases the life of the cutters.